Disabling basic_linear seems like a good idea but your config file sets the
cut-off at 128 Bytes for 64-ranks (the field you set to 8192 seems to result
in a message size of that value divided by the number of ranks).

In my testing bruck seems to win clearly (at least for 64 ranks on my IB) up
to 2048. Hence, the following line may be better:

131072 2 0 0 # switch to pair wise for size 128K/nranks

Disclaimer: I guess this could differ quite a bit for nranks!=64 and different
btls.